1. Field of the Invention
This invention relates to an image formation apparatus such as a copying machine or the like.
2. Description of the Prior Art
Copying machines use, as the light source for exposure, halogen lamps, fluorescent lamps, mercury lamps, xenon tubes, etc. In electrophotographic copying machines, fluorescent lamps and halogen lamps are most popular. To obtain copies of proper density, the charging, exposure and developing means must be individually adjusted because the conditions of these means have relations with the quality of copies. However, it is difficult for the user to adjust the charging and the developing means. Therefore, it has usually been the practice that the conditions of the charger and the developing device are fixed or may be adjusted only by experts such as service men and copies of good quality are obtained by varying the exposure amount in accordance with the conditions (for example, diazo and colored characters) of the image original, namely, adjusting the brightness of the optical image on the surface of a photosensitive medium.
There are the following two methods of adjusting the exposure amount:
a. to mechanically change the diaphragm blade.
b. to electrically vary the brightness of the light source itself.
The method a. has heretofore been often used because, in this method, it is entirely unnecessary to vary the quantity of light of the light source itself. A disadvantage of this method is that it requires a large space because of its being of a mechanical type, which in turn has led to the great complexity of the mechanical construction and difficulties in the adjustment thereof. Further, the lamp always puts out its maximum quantity of light and requires a maximum electrical power even when the exposure amount is small. This means great loss of the electrical power and shorter life of the light source. The method b., which comprises varying the brightness of the light source itself by varying the electrical power applied to the light source, requires the applied voltage to be varied. These are various methods of varying the voltage, such as the conduction phase adjustment, the adjustment effected with a variable voltage type auto-transformer opened, and the like, but owing to the recent remarkable advancement of the semiconductor technique, it has become possible to provide compact, simple and long-lived devices by using highly stabilized semiconductor control elements of high performance (triacs).
In this latter method, however, even when one thinks that one has adjusted the quantity of light to a predetermined level, the actual quantity of light (the density of the reproduced image) becomes irregular due to differences in environment, temperature and element characteristic and, where it is desired to finely adjust the predetermined quantity of light, a very cumbersome procedure and technique has been required of the operator.
In the recent copying machines, halogen lamps have become used as the light source for exposure, but the illumination of a halogen lamp is varied in proportion to about the cube of the applied voltage and therefore, when the voltage applied to the heater of the fixing device has been dropped by temperature control, it affects the halogen lamp. The usual halogen lamp requires a rising time of the order of 300 mS until its illumination reaches a predetermined value when the rated voltage for the lamp is applied thereto. Accordingly, fog may be created if copying is started with such rising time neglected. Moreover, if it is attempted to turn on the lamp with a low illumination by applying thereto a voltage below the rated voltage, more time will be required before the lamp reaches the desired illumination. Therefore, the timing for the exposure of the original must be determined in accordance with the low illumination turn-on which is slowest in rising. This leads to the need of the waiting time from the starting of the copying till the starting of the exposure of the original, which after all means a slower copying speed.
Further, irregularity of the rising time of the lamp itself resulting from the characteristic irregularity of the environmental conditions such as temperature and humidity, irregularity of the velocity of movement of the optical system and the original carriage, and irregularity of the sensitivity of the photosensitive medium attributable to its quality and environmental conditions--coact such that the electrostatic latent image corresponding to the end of the original is not formed appropriately and fog is sometimes created on the end of the copy image. If a waiting time is provided to compensate for the irregularity of the rising time, the copying speed becomes slower and slower to elongate the time required for the repeated copying.
Also, when copies are desired in haste, the operator often fails to confirm the position of the dial or effects the copying through wrong operation of the apparatus to produce useless copies of unsuitable density. Further, when the adjustment of the exposure is effected by adjusting the electrical power supplied to the light source, the color temperature of the light source may be varied and this, coupled with the spectral characteristic of the sensitivity of the photosensitive medium, has offered problems in image formation.
In an image formation apparatus using a short focus small image forming element array as the image forming optical system, it is actually very difficult or impossible to incorporate means such as a diaphragm blade in the image forming optical system, and it is extremely difficult to precisely limit the light flux between the original to be copied and the short focus small image forming element array or between such array and the photosensitive medium because the width of the effective light flux contributing to the image formation is very narrow, and thus it tends to cause deterioration of the quality of the copy image.
Also, in a copying machine of the variable magnification type, for example, the correction of the exposure amount when the magnification has been changed becomes a problem. For example, if reduced scale copying is effected at a magnification n.sub.2 only with the lens, etc. moved after copying has been effected at a magnification n.sub.1 and under a proper exposure, the then exposure amount will become excessive and readjustment will be required in order to provide a proper exposure amount.
Also, to obtain proper copies at each copying magnification, adjusting means for use by the operator becomes necessary for varying the quantity of light in accordance with the conditions (e.g. diazo and colored characters) of the original to be copied.
However, in a copying machine of the variable magnification type, the position of the lens is displaced in accordance with the change of the copying magnification and therefore, not only the construction for varying the diaphragm blade becomes very complex but also the mechanical construction for indenpendently adjusting the amount of correction in each magnification becomes complicated.